The present invention relates to a closed cycle refrigeration system for use in refrigerated installations. In particular, it relates to an oil return means which permits crank case oil from the compressors of such a refrigeration system to be returned to the various compressor crank cases dependent upon the requirements for oil flow thereto.
Commercial refrigeration systems operate through the use of compressors which draw in refrigerant gas through a suction line, compress the gas, and then discharge high pressure gas to a condenser means where it is condensed to a liquid. Thereafter the liquified refrigerant is put through an expansion valve into one or more evaporator coils which are the low temperature elements in the closed circuit. Thereafter the refrigerant gas formed during evaporation of the liquid refrigerant is returned to the compressor by a suction line. In many commercial systems multiple compressors are employed with the same series of evaporator coils in order to provide for variable refrigeration loads which are imposed upon the refrigeration system. In these systems the refrigerant gas from the evaporators is gathered in a suction manifold which is in turn connected to separate suction lines. Such variable refrigeration loads can occur due to frequency of use of the refrigerated installations, for example refrigerated display cases in supermarkets or can occur due to variable ambient heat and humidity conditions. In such systems the multiple compressors are controlled by a control means which is sensitive to the variable refrigeration loading conditions.
In the refrigeration art it is considered important to distinguish between systems operating with only a single compressor and those operating with multiple compressors due to the greater complexity of refrigerant flow lines and control means in the case of the latter. Another reason to make this distinction is that commercially employed compressors are driven by motors connected to crank shafts which are in turn connected with one or more pistons which perform the compression function. The operation of the crank shafts and pistons requires a lubricating oil flow which can be easily controlled in the situation where a single compressor is employed in a refrigeration system. When multiple compressors are to be used in a refrigeration system in order to accommodate variable refrigeration loads, the handling of the flow of lubrication oil becomes somewhat more involved.
During the operation of a refrigeration system having multiple compressors, lubricant oil in the crank case oil bath is entrained in the refrigerant gas flow into the high pressure flow line leading away from the compressors. This oil then coats the entire inner surface of the refrigeration system and accumlates in certain of the components thereof. Provision must then be made for returning the accumulated lubricating oil to the crank cases of the compressors in a manner which provides an optimum supply of oil to each of the compressors. In refrigeration systems where some of the compressors are only operated periodically in response to variable refrigeration loading, the return flow of lubricating oil can be a particular problem. If the return oil flow occurs continuously during the operation of the system which is powered by less than all of the multiple compressors, the crank case of the non-operating compressor can overfill with oil which then can cause compressor damage during start-up.
A widely used commercial type of refrigeration system operating with multiple compressors connects the individual suction lines to each of the compressors from the bottom portion of the suction manifold in the system so that the oil layer in the bottom thereof is siphoned off continuously to each of the compressors. This type of continuous oil flow through siphoning results in the above described problem.
Other commercial systems utilize oil separators to separate most of the oil from the high pressure discharge lines connected to the compressors and to then return this oil to the compressor crank cases through oil floats which are used to allow the proper amount of oil to flow to individual crank cases. Oil reservoirs have been designed with sight glasses to assure that the systems can be checked for adequate oil levels depending upon different refrigeration loads.
In the art of refrigeration systems the use of flooded types of evaporators causes a particular problem at partial refrigeration loads in that the gas velocities through the evaporators are too low to entrain and return lubricating oil to the compressor. Normally at the termination of a defrost cycle the oil in the evaporator will be driven back to the suction manifold. This oil will flow in equal proportion to all the compressors, including the idle ones. Therefore high oil levels may develop in the idle compressor(s). This may result in damage of these idle compressor(s) on start-up. For this reason it is necessary to evolve some type of means to separate the entrapped oil and to return such oil to the compressor crank case.
U.S. Pat. No. 2,145,721 to Hall describes a flooded evaporator which has a header formed therein with a suction conduit for the transport of oil mixed with vaporised refrigerant. Such an evaporator is constructed for use with a single compressor.
Another problem which has arisen in the refrigeration art is that the lubricating oil which is entrained with the refrigerant gas flow collects in certain components within the refrigeration system and must then be fed back into the refrigerant flow line leading into the compressor in order to correctly proportion the lubricating oil with respect to the flow of the refrigerant. Such systems operate for recombining the lubricant oil with the refrigerant in a way to preserve a workable proportion of the two components. These systems have been involved with refrigeration systems having single compressors. Representative of the solutions to this problem are U.S. Pat. Nos. 2,021,691 to Kenney; 2,121,253 to McGuffey; 1,899,378 to Zouck et al.; and 3,111,819 to Williams.
One component of a refrigeration system in which lubricating oil accumulates is an anti-slugging tank such as shown in U.S. Pat. No. 3,180,567. An oil flow tube is placed within the tank in order to provide a passage for the accumulated lubricating oil into the compressor casing. Another system of a similar type is shown in U.S. Pat. No. 3,177,680 to Rasovich et al.
U.S. Pat. No. 3,438,218 to O'Neil discloses a flooding type evaporator which contains a centrally disposed tube having baffles thereon through which lubricating oil is entrained with the refrigerant gas flow in order to provide for the lubricating oil flow to compressor 2.
U.S. Pat. No. 2,663,164 to Kurtz shows a tube 15 for interconnecting two compressors located at different vertical positions so that a constant oil level is maintained in the higher of the two compressors. U.S. Pat. No. 2,042,558 to Steenstrup shows an arrangement for collecting lubricating oil which is lighter than the liquid refrigerant from the surface of the liquid in a flooding type evaporator, and hence is similar to the above-referenced Hall patent. U.S. Pat. No. 2,614,402 to Swart shows yet another means for returning lubricating oil to a compressor 2.
U.S. Pat. Nos. 4,141,223 and 4,142,380, both to Dyhr and Nissen, show an encapsulated compressor with a centrifugal separator 22 attached thereto which accumulates both liquid refrigerant and oil in its bottom section.
U.S. Pat. No. 3,276,215 to McDonell shows a multiple compressor system which has a common oil return line 22 which feeds oil to all of the compressors as a group.
The above referred to patents show that there has not been a sufficient appreciation of the problem of providing for intermittent oil flow to a series of multiple compressors which are operated periodically and individually in response to variable refrigeration loading. Such oil return means must also operate intermittently in order to prevent the accumulated oil in the system from flowing continuously into one or more of the compressors which is not then in operation. If provision for such intermittent oil return is not made, the crank case of the non-operated compressors will fill with oil to an unacceptably high level. Such means for intermittent oil return flow should be simple in construction and avoid mechanically operating valves. The above patents have not recognized the oil return flow problem and have not generated solutions to this problem.